It is known that for the above-mentioned applications, bellows are used that are made with austenitic stainless steels or superalloys, such as INCONEL for example (for high-temperature applications), having double-walls, a radially outer one and a radially inner one, each one sized to resist the design conditions independently of the other. A spacer, composed of fine-mesh metal netting for example, is inserted between the two walls in order to ensure radial clearance between the two walls that, in the case of failure in the inner wall or the associated longitudinal weld of the latter allows passage of pressure of the fluid contained in the bellow, which is detected by a specially provided instrument located on the straight end, devoid of undulations, of the bellows itself, known as the collar.
In order to allow testing of the double-wall bellows, it is evident that the end rims of the collars, which in use will be welded to the respective end rims of the pipes to be connected via the hydraulic coupling, of which in use the bellows forms part, must be fluid-tight sealed before fitting the bellows itself.
It is clear that user requirements are directed towards sealing systems that cancel, or reduce to the minimum, the risk of leakage after phase of circumferentially welding the bellows to the piping. To this end, the most requested sealing system is that which uses the resistance welding process.
Thus, the most commonly used sealing method exclusively employs manual TIG butt-welding of the rim, with or without the aid of filler metal, in function of the thickness of the walls of the bellows.
Nevertheless, although guaranteeing a high level of reliability, the above-illustrated sealing method does not exclude leakage risks after welding the bellows to the pipes to be connected (a process known as “bellows-pipe welding”), as during the sealing phase it does not allow a weld bead to be obtained on the rim having sufficient depth in the base material (in the axial direction), in terms of fusion, to exclude subsequent leakage without triggering metallurgical and mechanical phenomena that would negatively impact the final characteristics of the coupling.
Secondly, the above-described known art requires a significant amount of time to be carried out (several hours) and the carrying out of monitoring tests for possible leaks.